The invention relates to jacks for tensioning stranded or wire (rods) cables contained in sheaths embodied in concrete structures which are to be prestressed with said cables. The known jacks generally provide for a cylinder-piston unit having an annular cross section to allow for a central zone not to be utilized by the hydraulic system but necessary to receive the end sections of the cables or strands protruding from the anchoring head to which the cables or strands are to be anchored after the tensioning performed by the jack. Said central zone must have its cross sectional area of the same magnitude as that of the anchoring head area or at least of the part of said head where the anchorages of the wires or strands are located. All this brings about remarkable transverse outer dimensions and a heavy weight for the jack.